This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Photolithography is a process used in the fabrication of integrated circuits, which entails using light to transfer a pattern from a “mask” to a “photoresist” on a substrate. A series of chemical treatments are then applied in order to etch a desired pattern into the substrate. Improvements in photolithography have advanced integrated circuit manufacturing over the last several decades. The most important advance is the gradual reduction of the wavelength of the illumination source of the photolithography tool: from the 436 nm line of a mercury lamp to the 193 nm light of an ArF laser. By incrementally improving the photolithography process, features as small as 45 nm can be manufactured in high volume using 193 nm photolithography. Immersion 193 nm photolithography is presently deployed for the manufacturing of even more advanced technology nodes. To print even smaller features, several technologies have been considered, such as extreme ultraviolet lithography (EUVL). The development, however, has been delayed due to several difficulties, in particular the difficulty in designing a light source with sufficient brightness. Consequently, multiple-patterning techniques have emerged as an alternative. However, multiple-patterning achieves a fine feature size at a significant cost. In its simplest form, double-patterning, two separate masks and two exposure steps are required to achieve what a single mask and one exposure step used to accomplish, leading to a much lower throughput and significantly higher cost.